大口徑單樁基礎為離岸風機常用之基礎型式，其主要設計控制載重來源為來自於風、波、流之側向力。Kuo (2008)與Achums et al. (2009)提出土壤勁度衰減模型，以動態三軸剪力試驗求取土壤受長期反覆軸向應力產生的應變量及割線模數，並結合有限元素樁土互制模型，評估基樁受極限設計條件 (ULS&SLS)下反覆側向作用力造成之樁身變形量。由於該模型可於離岸風機基礎設計實務應用，因此BSH (2012)規範建議於離岸風機基礎設計時，可以該方法計算大口徑單樁基礎受長期反覆側向力之變形量。當海上結構物受長期反覆外力作用時，除了考量ULS與SLS條件外，亦須考量極限疲勞設計條件(FLS)，以及自然振動頻率的變化。於FLS條件下，疲勞載重將由風機運轉時的動態響應決定，基礎勁度則由FLS作用力條件下加載-卸載之受力變形回彈反應取得。Bhattacharya et al. (2013b)說明砂質土壤之回彈模數隨反覆作用次數增加產生硬化反應，提高基礎勁度，即當單樁基礎於極限疲勞設計條件 (FLS)下受微小振幅側向力作用時，樁周土壤的回彈模數將隨反覆作用次數累積而提高，樁土互制系統之勁度將隨反覆側向載重而增加。為評估基礎勁度隨風機運轉時反覆載重下之變化，本研究以動態三軸剪力試驗，取得土壤受反覆應力條件下之回彈模數，以室內試驗成果結合有限元素樁土互制模型，計算大口徑單樁基礎於微小振幅反覆載重下之基礎勁度，提供離岸風機於FLS條件計算整體結構之自然振動頻率與疲勞損傷。

Monopile foundation is commonly used as the foundation of offshore wind turbines. The lateral loads come from wind, wave and current applied on the monopiles and dominate the behavior of the monopiles. The Stiffness Degradation Method (SDM) proposed by Kuo (2008) and Achmus et al. (2009) presents a methodology to calculate the deformation of monopile under long-term cyclic lateral load. The pile stiffness under ULS (Ultimate Limit State) and SLS (Serviceability Limit State) can be determined from the loading conditions and the deformations responses obtained from SDM. For the FLS (Fatigue Limit State), the fatigue loads need to be determined from dynamic responses of offshore wind turbines and the foundation stiffness is determined with the load-deformation responses in the loading-unloading cycle. A method enlightened by SDM for calculating the foundation stiffness of monopile foundation under FLS is presented in this study, which is cooperated with the resilient modulus of cohesionless soil obtained from cyclic triaxial test results and a numerical model programmed with commercial finite element software. The elastic deformation of monopile under cyclic lateral load can be quantified by the method presented in this study. A decreasing elastic deformation of monopile is found with the increasing number of loading cycles. An increasing foundation stiffness of monopile presents with the increasing number of loading cycles. The change of foundation stiffness needs to be considered in the fatigue load analysis and lifetime evaluation of offshore wind turbine support structures

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